Quasispecies and the implications for virus persistence and escape

Background: In the 1970s Manfred Eigen and colleagues proposed a new model of molecular evolution to explain adaptability and rapid evolution of simple replicons, as those that probably populated the earth at the onset of life. This model of evolution placed emphasis on mutant generation, to the point of invalidating the concept of wild-type genomes as a defined sequence of nucleotides. In striking similarity with the proposals for such early replicons, present-day RNA viruses consist of complex distributions of nonidentical but closely related genomes termed quasispecies. Objectives: To discuss indeterminations inherent to a quasispecies structure and to the analytical procedures to define it, biological implications of quasispecies, and the need to take into account this type of population structure, in order to design effective strategies to prevent and control diseases caused by highly variable viruses. Results: Quasispecies have many biological implications, extending from viral pathogenesis to the emergence of new pathogens, rapid antigenic variation, and alterations in cell tropism, virulence, host range and viral gene expression. Conclusions: Diseases caused by highly variable RNA viruses prove very difficult to control and vaccine development against such viruses are largely unsuccessful. It is important to understand quasispecies composition and dynamics, as quasispecies are an important step in the natural history of RNA viruses.